1. Field of the Invention
The present invention relates to an electronic circuit and a method for making an electronic circuit, and in particular an electronic circuit at least partially electromagnetically shielded.
2. Description of the Related Art
An electronic device mostly includes and operates an electronic circuit constituted by including circuit elements such as semiconductors and passive components on a circuit board contained in a housing. In some cases, such an electronic circuit constituted on the circuit board is entirely or partially shielded electromagnetically in order to reduce an effect of noise coming from the outside or to suppress self-generated noise. It is further important that a radio communication device such as a mobile phone configured to emit electromagnetic waves to the air deal with noise by means of an electromagnetic shield (simply called “shield” hereafter).
In a case where an electronic circuit constituted on one circuit board is divided into a plurality of blocks, it may be necessary to selectively shield the electronic circuit on a block-by-block basis. According to a known method, in such a case, partition plates are used for shielding the plural circuit blocks from one another. Further, a method for providing a shield frame on a board so as to surround a portion to be shielded and for fastening a shield cover to a fringe of the shield frame so as to cover the portion to be shielded is recently used in order to improve assembly work efficiency, as disclosed in, e.g., Japanese Patent Publication of Unexamined Applications (Kokai), No. 2004-235501.
According to JP-A-2004-235501, a fringe of a shield case frame (shield frame) facing a printed board is bent to the inside and a portion of the fringe is partially removed and soldered so that the shield frame is fixed to the printed board, and that space efficiency is raised as the fringe is bent to the inside.
Other than an electronic circuit constituted on a circuit board, a flexible printed board for connecting among circuit boards or between the circuit board and other components for signal transfer becomes a noise source in some cases. As signal transfer speed is getting higher in recent years, it becomes more important that the noise be properly dealt with around the flexible printed board, as disclosed in, e.g., Japanese Patent Publication of Unexamined Applications (Kokai), No. 2006-303081.
According to JP-A-2006-303081, a connection between a connector provided on a circuit board and a flexible printed board connected to the connector is covered by a support plate made of a ferrite core, so that a portion of the flexible printed board connected to the connector is shielded.
According to JP-A-2004-235501 described above, as the shield case frame is provided in such a way as to surround electronic parts to be shielded arranged on the circuit board, no flexible printed board can be provided for connecting the inside and the outside of the shield case. Thus, there is a problem that the configuration of JP-A-2004-235501 cannot be used to deal with the noise of a flexible printed board.
According to JP-A-2006-303081 described above, as only the connector portion connecting the flexible printed board and the circuit board is shielded, there is a problem that a transfer line portion of the flexible printed board cannot be shielded.
If a transfer line portion of a flexible printed board needs to be shielded, measures are ordinarily taken such that the portion is provided with silver paste. There is a problem, however, that silver pasting is usually expensive and thus can hardly be applied to commodities to be sold.